2002 Volume 42 Issue 6 Pages 636-644
The evolution of microstructure during a simulation of the thin slab direct rolling process has been studied on two low carbon steels, microalloyed with V-N and V-Ti-N. The steels were examined using optical microscopy, analytical transmission electron microscopy (TEM) and energy dispersive X-ray (EDAX).
After the 4th rolling pass, in a five pass schedule, the initial coarse austenite grain size (≈1 mm) was reduced to about 50 μm in Steel V-N and 22μm in Steel V-Ti-N. The average ferrite grain size in the final strip was slightly smaller in Steel V-Ti-N (4.8-6.6 μm) than in Steel V-N (5.3-7.2 μm). For Steel V-N, VN was only observed after 1 050°C equalization, but it was not found after 1 200°C and 1 100°C equalisation. For Steel V-Ti-N, V-Ti(N) particles formed during casting and during equalization for all the equalization temperatures (1 200°C, 1 100°C and 1 050°C). AlN particles precipitated in Steel V-N only during 1 050°C equalization and were often associated with MnS or MnS and VN. No AlN was detected in Steel V-Ti-N. Fine V containing precipitates (<10 nm) were observed in the final strip for both of the steels, but the frequency of the fine particles was lower in Steel V-Ti-N than in Steel V-N. The fine precipitates in the final strip make a major contribution to dispersion strengthening. High strength (LYS≈460-560 MPa) with good toughness and good ductility were achieved in the steels, which are competitive to similar products made by conventional controlled rolling. However, the addition of Ti to the V-N steel decreased the yield strength due to formation of V-Ti(N) particles in austenite, which reduced the amounts of V and N available for subsequent V rich fine particle precipitation in ferrite.